uniform float timestep = 2.0;
void main()
{
    vec3 pos = gl_Vertex.xyz;
    vec3 vel = gl_MultiTexCoord0.xyz;
    vec3 pos2 = (pos - vel*timestep);   // previous position

    gl_Position = gl_ModelViewMatrix * vec4(pos, 1.0); // eye space
    gl_TexCoord[0] = gl_ModelViewMatrix * vec4(pos2, 1.0);

    gl_FrontColor = gl_Color;
}